Twins

So I had my first u/s yesterday which I thought was 8 weeks . OB says I'm measuring 6 weeks 4 days. There are 2 sacs with embryos but only 1 heartbeat was detected. I go back in 1 week for repeat scan. Dr. Doesn't sound hopeful that the second baby will make it. Has anyone been through this ? Is it possible the heartbeat will show up next week